The present invention relates to an antenna assembly comprising a whip antenna and a helical antenna mounted on a top end of the whip antenna and, in particular, to a mounting structure of the helical antenna to the whip antenna.
In the prior art, an antenna assembly of the type described has been used in a radio communication device such as a portable communication terminal set, especially a mobile telephone terminal set. The antenna assembly is usually extendably and retractably mounted to a casing or a housing of the terminal set as disclosed in JP-A-3 245603 (Reference I).
In Reference I, the terminal set has a housing or enclosure enclosing transmitting and receiving electrical circuitry. The antenna assembly comprises the whip antenna or an antenna rod which is slidably supported in a support mounted to the housing. The support is made of conductor and is connected to the electrical circuitry. The whip antenna is provided with a stopper or a conductive ring fixedly mounted on a lower or an inner end. When the whip antenna is in an extended position, the stopper is brought into contact with the support, so that the whip antenna is connected to the electrical circuitry through the stopper and the support. The whip antenna comprises a conductive rod covered with a dielectric sleeve or sheath. The whip antenna or the conductive rod has an electrical length of a quarter wavelength of a predetermined frequency.
The helical antenna or an antenna coil is enclosed in a dielectric cap and is carried on a top end of the whip antenna. The dielectric cap is provided with a conductive sleeve at a lower end electrically connecting with the helical antenna. The conductive sleeve is fitted onto the top end of the whip antenna and fixed thereto by caulking or deforming the conductive sleeve together with the dielectric sleeve. The helical antenna is connected to the conductive rod and also has an electrical length of a quarter wavelength of the predetermined frequency. Therefore, the antenna assembly has a half wavelength of the predetermined frequency.
When the antenna assembly is in a retracted position, the whip antenna is retracted in the housing with the helical antenna being connected with the electrical circuitry through the conductive sleeve and the support. Thus, the helical antenna is used for short-range operation of the terminal set. In this condition, the whip antenna is in the housing and, therefore, does not serve for receiving a radio signal.
For a long-range operation, the antenna assembly is pulled out by manually handling the cap into the extended position where the stopper is brought into contact with the support. Thus, the antenna assembly serves as a half-wavelength antenna.
An assembly of the helical antenna and the dielectric cap with the conductive sleeve will be referred to as an antenna top.
In order to insure that the whip antenna is disabled when the antenna assembly is in the retracted position, the terminal set may have a coaxial support in which the whip antenna is positioned in the retracted condition. This is disclosed in GB 2,257,836 A (Reference II) and JP-A-5 243829 corresponding thereto.
There is also known in the prior art, for example, U.S. Pat. No. 5,204,687 (Reference III) and JP-B-2646505 (Reference IV) another structure of the antenna assembly where the conductor rod of the whip antenna is not electrically connected with the helical antenna but is insulated therefrom. In the structure, the whip antenna is reliably disabled in the retracted condition without use of a special support structure such as the coaxial structure. The whip antenna only serves for receiving the radio signal in the extended condition because the helical antenna is no longer connected to the support. This structure of the antenna assembly will be referred to as a "separate type", because the whip antenna is electrically separated from the helical antenna.
In detail, Reference IV discloses a dielectric joint member of a generally rod shape which is secured at one end thereof to the top end of the conductor rod of the whip antenna. The joint member is partially covered with the conductive sleeve and is fitted at the other end portion with a coil bobbin. A helical coil or the helical antenna is wound on the coil bobbin and is connected to the conductive sleeve. The dielectric cap covers on the coil bobbin, the helical coil and the top end portion of the conductive sleeve together by, for example, the plastic molding to form the antenna top.
In the above, the conductive sleeve and the top end of the conductive rod of the whip antenna are fixed to the joint member by the insulation molding of the joint member when the conductive sleeve and the top end of the conductive rod are inserted into a mold.
The antenna top is manipulated in an operation for bringing the antenna assembly into the retracted condition or the extended condition. Due to rough handling of the antenna top in the operation or in the habits of users, the antenna top may often be subjected to unusual rotating torque or undue flexural force, so that the antenna assembly is easily broken or cut by twisting at the point near the boundary between the conductive sleeve and the joint member.